This invention relates to a process for preparing a diester of carboxylic acid. More particularly, it, relates to a process for preparing a diester of carbonic acid which comprises bringing carbon monoxide into contact with an ester of nitrous acid selectively.
A diester of carbonic acid is an extremely available compound as an organic synthesis starting material for medicine and herbicides and also as an intermediate for preparation of a polycarbonate or urethane.
As a process for preparing a diester of carbonic acid, there have been practiced a method of reacting phosgene and an alcohol from extremely old times, but phosgene has extremely high toxicity and hydrochloric acid is a by-produced by the reaction of phosgene and an alcohol so that great care must be taken for a material to be used in an apparatus. Thus, it has been desired to develop a method using no phosgene.
Accordingly, a method for synthesizing a diester of carbonic acid from an alcohol and carbon monoxide has been researched various fields as a process of using no phosgene, and has been proposed (for example, Japanese Provisional Patent publications No. 75447/1985, No. 72650/1988 and No. 38010/1988, and WO-87/7601). These are methods in which a diester of carbonic acid is synthesized by oxygen oxidation reaction of an alcohol and carbon monoxide using copper halide or palladium halide as a catalyst, but involve the problems that a selectivity of a diester of carbonic acid based on carbon monoxide is low since carbon dioxide is by-produced and purification of a diester of carbonic acid is complicated due to formation of water.
Thus, as a method for improving the conventional ones, it has been proposed, in Japanese Provisional Patent Publication No. 181051/1985, a method of preparing a diester of carbonic acid which comprises reacting a nitrite and carbon monoxide in a vapor phase in the presence of a solid catalyst in which a platinum group metal or a compound thereof is carried on a carrier, and an oxidizing agent with an amount of 10 mole % or more as O.sub.2 per carbon monoxide.
However, nevertheless an oxidizing agent such as oxygen co-exists with such a ratio to carbon monoxide in order to prevent by-production of a diester of oxalic acid, a significant amount of a diester of oxalic acid is by-produced so that a selectivity of a diester of carbonic acid is low and also the reaction rate is not sufficient. Also, a used range of a nitrite in "a mixed gas comprising a nitrite, carbon monoxide, an alcohol and oxygen" exceeds an explosion limit which is a problem in safety whereby the method is not an industrially sufficient one.
The process for preparing a diester of carbonic acid using a nitrite conventionally known is insufficient in reaction rate between carbon monoxide and a nitrite as mentioned above, and also a selectivity of a diester of carbonic acid is low so that there is a defect that a purification processing of the diester of carbonic acid formed becomes complicated. Further, the concentration range of nitrite in the reaction system exceeds an explosion limit so that there is a problem of accompanying danger in operations.